A substantially all-silicon capacitance-type pressure transducer is described in U.S. Pat. No. 3,634,727. That device is formed of two disks of silicon hollowed centrally, coated with insulating oxide, and then joined together either by metallizing the oxides followed by brazing, or simply with low melting temperature glass. Such device is on the order of 1" diameter and inherently has a relatively high ratio of unwanted static parasitic capacitance to variable capacitance as a function of pressure.
The method of bonding the two silicon disks together in the aforementioned patent is complex and costly. Additionally, the device of the aforementioned patent requires the processing of two wafers to provide only a single capacitive pressure transducer. Obviously, the utilization of low cost silicon pressure transducers dictates the need to fabricate them utilizing mass production techniques. For instance, processing a pair of larger wafers to make a significant number of smaller capacitors (on the order of one-half cm in the largest dimension) could significantly reduce the cost. However, the assurance of a complete seal between the two portions of each of the transducers formed on a wafer is mandatory.
A better method of forming silicon-to-silicon seals utilizes sputtered borosilicate glass. The glassed silicon portions are attracted to each other by a DC field in a vacuum, at a temperature on the order of 500.degree. C. This is disclosed in NASA Tech Brief B74-10263, January 1975, entitled "Low Temperature Electrostatic Silicon-To-Silicon Seals Using Sputtered Borosilicate Glass".
The advantage of using silicon as the base material for cavitied, capacitive pressure transducers is because it allows external electrical connection to internal capacitor plate surfaces, without requiring complex mechanical structure. Coupled with well known microcircuit technology for processing silicon, and the field-assisted bonding technique of the NASA Tech Brief, the opportunity for mass production of extremely small, sensitive, capacitive pressure transducers, in a highly reproducible fashion, becomes apparent. However, when capacitive transducers are on the order of a half cm in their largest dimensions, the capacitance is very small unless the two capacitive plates are extremely close together. The closeness of the two capacitive plates has been found to interfere with the use of field-assisted bonding in making extremely small, silicon capacitive pressure transducers.